(1) Field of the Invention
The present invention relates to a method and system for transmitting a signal and receiving and reproducing the signal in a digital broadcasting situation, and a transmitting apparatus and a receiving and reproducing apparatus used in a digital broadcasting situation, suitable for a system for receiving digital satellite broadcasting by a receiver carried in a mobile unit.
(2) Description of Related Art
Recently, in parallel with putting digital satellite broadcasting into practice, there have been developed and proposed various image data compression system, audio data compression system and the like. Also, there are discussed systems of various manner for receiving digital satellite broadcasting by a receiver carried in a mobile unit.
When a satellite broadcasting is to be received, in general, it is necessary to prepare a parabolic antenna for receiving the radio wave of which frequency is allocated by the authority. Therefore, it was unrealistic for a user of the mobile unit to prepare such. an antenna to receive the satellite broadcasting. However, since an S-band (2.6 GHz band) frequency band, which is particularly unsusceptible against rain, is allocated for mobile units to receive the satellite broadcasting, it becomes realistic, for the mobile user for whom proper receiving means was not available, to receive the satellite broadcasting.
When broadcasting is provided for mobile users, a broadcasting receiver receives and reproduces the broadcast digital signal while it is carried in or with moving bodies such as a vehicle, a person, a ship or the like. Therefore, if these moving bodies pass an area near a building, a tree, a bridge, a tunnel or the like, the radio wave transmission path tends to suffer from radio wave cut due to an obstacle. Further, if the receiver is utilized in a stationary fashion, the radio wave transmitted from the broadcasting station also will suffer from radio wave cut due to a moving obstacle, with the result that the broadcast program will suffer from reproduction interruption. Therefore, the receiving side of the broadcast system cannot be provided with a satisfactory broadcasting service due to the disturbance in picture reproduction or audio reproduction. With regard to the disturbance in picture reproduction, if a compensation technology such as a still image retention or the like is effected in the receiving apparatus, uncomfortable feeling of a viewer can be relieved effectively with a relatively easy effort, and it is possible to reproduce images at an acceptable level in visual sensation standpoint. Conversely, if sound reproduction is interrupted on the receiving apparatus, uncomfortable feeling of a listener cannot be relieved effectively with a relatively easy effort even if a countermeasure such as muting or the like is taken. Particularly when an audio broadcasting is provided for mobile users and a driver of a vehicle is paying his/her attention to the reproduced sound intensively, the uncomfortable feeling due to the sound break will affect more badly. Thus, there is a great need in improving the audibility under such situation. Further, under broadcasting situation in which point-to-multipoint communication is carried out, it is substantially impossible to provide a retransmission service for each of the viewers or listeners upon request from receivers of these viewers or listeners. Therefore, it is a responsibility for the communication system to continue the reproduction of the broadcast program without issuing or responding to the retransmission request even if the radio wave cut is brought about.
In order to avoid the above problems, the following countermeasures have been taken.
{circle around (1)} A gap filler (radio wave relaying station or retransmission equipment) or the like is installed at a shadow area of a building, a tree, a bridge or the like to which radio wave is not satisfactorily supplied. Alternatively, a plural number of broadcasting satellites remote from one another are utilized for transmitting radio waves.
{circle around (2)} Under estimation of time duration of radio wave cut expected when broadcasting is interrupted due to an obstacle in the radio wave path, degree of interleave is decided so that the estimated time duration of radio wave cut can be sufficiently recovered. Thereafter, receiving error in a burst fashion is spread so that error is distributed in a random fashion and then the random error is corrected by using an error correction scheme such as Viterbi coding or Reed-Solomon coding.
{circle around (3)} A plurality of channels are prepared for broadcasting a program at the same quality, and each of the signals of the respective channels are transmitted so as to have a time lag larger than the expected radio wave cut time relative to another, whereby any of the transmitted signals with the time lag among them can be successfully received on the receiving side with the result that the broadcast program can be continuously reproduced.
However, according to the countermeasure numbered one, in order to eliminate all areas in which radio wave is not satisfactorily supplied, great number of gap fillers are necessary, which fact necessitates large amount of investment for supplying and installing the equipment. Further, it is difficult to utilize a plural number of satellites from a practical viewpoint. According to the countermeasure numbered two, it is difficult to correct completely error caused from the radio wave obstacles by the scheme composed of interleave and error correction code. When a program is broadcast in a digital manner, the broadcast program is encoded and compressed in every frame. Therefore, even if a frame contains only one bit error, the frame will not be decoded. Furthermore, according to the countermeasure numbered three, if a broadcasting program of the same quality is broadcast by using a plural number of channels, although it offers a satisfactory solution against the radio wave transmission problem, frequency bands cannot be effectively utilized in spite of the fact that the frequency band is a limited resource.
The present invention is made in view of the above aspect. According to the present invention for a system in which digital satellite broadcasting using S-band for example is received by a receiver carried in a mobile unit, when a time diversity system in which identical data are transmitted a plural number of times, the invention is employed in a mobile satellite broadcasting system to cope with an obstacle in the broadcast radio wave transmission, a plural number of high quality signals, which require the same bandwidth, are not prepared for transmission from the transmitting side, but a low quality signal data, which requires relatively small bandwidth, is transmitted before the high quality signal data is transmitted. When a receiving side receives the broadcasting program, the receiving side selectively reproduces the low quality signal data having the small transmission bandwidth to recover the reproduction of the broadcasting program with the high quality signal. According to the method of transmitting a signal and receiving and reproducing the signal in a digital broadcasting situation, the system for transmitting a signal and receiving and reproducing the signal in a digital broadcasting situation, and the transmitting apparatus and the receiving apparatus used in a digital broadcasting situation of the present invention, since the low quality signal having a relatively small bandwidth is selectively utilized for recovering the program data transmission with the high quality signal, the frequency band for broadcasting can be effectively utilized, and a number of broadcasting programs can be transmitted within a limited width of frequency band. Furthermore, investment on the broadcasting equipment such as a gap filler or the like can be minimized.
According to the proposed method of the present invention in which a signal is transmitted from a transmitting side and the transmitted signal is received and reproduced on the receiving side in a digital broadcasting situation, the method is carried out in such a manner that, on the transmitting side, a high quality signal is created based on broadcast digital data and at least one low quality signal is created based on the same broadcast digital data so that the quality of the low quality signal is low with respect to that of the high quality signal, and the high quality signal and the low quality signal are transmitted so that one of the signals has a time lag relative to the other, and on the receiving side, the high quality signal is reproduced when a normal receiving mode is effected while the low quality signal is reproduced when a broadcast hindrance mode is effected.
According to the above method, even if reproduction based on the high quality signal is failed on the receiving side, the reproduction of the broadcasting program can be recovered by reproducing the low quality signal which requires relatively small bandwidth to be transmitted. Therefore, the frequency band allocated for broadcasting the programs can be effectively utilized and thus number of programs allowable to be broadcast can be increased. Moreover, cost for system management per unit program can be reduced. In addition, according to the above arrangement, since data transmitted plurality of times need not be completely identical to one another, it is possible to utilize the frequency band for broadcasting effectively while the advantage of time diversity system that investment in providing gap filler or the like can be minimized for carrying out broadcasting.
According to the method of the present invention, on the transmitting side, a plurality of low quality signals different in quality from one another may be prepared as the low quality signal, and the high quality signal and the plurality of low quality signals are transmitted so that each of the signals has a time lag relative to another. The low quality signal may be transmitted before the high quality signal is transmitted. The high quality signal may be transmitted by using a first channel band while the low quality signal may be transmitted by using a second channel band of which width is narrower than that of the first channel band. The high quality signal is transmitted by using a first channel band while each of the low quality signals may be transmitted by using a second channel band of which width is narrower than that of the first channel band. Furthermore, each of the channel bands has a width different from one another.
According to the above proposed methods, the low quality signal created based on the broadcast data identical to that from which the high quality signal is created, is reproduced upon recovering the reproduction of the high quality signal. Therefore, even if the radio wave of the high quality signal is interrupted due to any obstacle upon receiving the signal, the reproduction of the broadcast program can be recovered and continued. Accordingly, a listener of the program can enjoy the program without uncomfortable feeling. Moreover, according to the above proposed methods, the frequency band allocated for broadcasting the programs can be effectively utilized and thus number of programs allowable to be broadcast at a time can be increased. Accordingly, cost for system management per unit program can be reduced. In addition, since the frequency band for broadcasting can be effectively utilized, it is possible to take the advantage of system of time diversity that investment in providing gap filler or the like can be minimized while broadcasting can be carried out.
In addition, when transition is brought about from the normal receiving mode to the broadcast hindrance mode on the receiving side, the high quality signal and the low quality signal may be processed and reproduced so that the transition is effected smoothly in terms of quality of the high quality signal and the low quality signal. Further, on the receiving side, after the broadcast hindrance mode takes place, transition to the normal receiving mode may be prohibited for reproducing the high quality signal until the high quality signal is successfully received for a predetermined time duration or predetermined number of frames.
On the receiving side, if the low quality signal is not received but the high quality signal is received under condition that the broadcast hindrance mode is effected, low quality data may be created from the high quality signal to recover the reproduction of the low quality signal. Further, when transition is brought about from the broadcast hindrance mode to the normal receiving mode on the receiving side, the high quality signal and the low quality signal may be processed and reproduced so that the transition is effected smoothly in terms of quality of the high quality signal and the low quality signal.
According to the above arrangement, broadcast data can be transmitted and received at a satisfactory quality level from practical viewpoint by a simple arrangement, with the result that a number of programs simultaneously broadcast can be increased, and cost for system management per unit program can be reduced. In addition, since the frequency band for broadcasting can be effectively utilized, it is possible to take the advantage of system of time diversity that investment in providing gap filler or the like can be minimized while broadcasting can be carried out.
According to the present invention, there is also provided a system composed of a transmitting apparatus for transmitting a signal and a receiving apparatus for receiving and reproducing the signal transmitted from the transmitting apparatus, both of the apparatus working in a digital broadcasting situation, wherein the transmitting apparatus comprises high quality signal creating means for creating a high quality signal based on broadcast digital data, low quality signal creating means for creating at least one low quality signal based on the same broadcast digital data so that the quality of the low quality signal is relatively low with respect to that of the high quality signal, and transmitting means for transmitting the high quality signal created by the high quality signal creating means and the low quality signal created by the low quality signal creating means so that one of the signals has a time lag relative to the other, and the receiving apparatus is arranged for receiving the high quality signal and the low quality signal transmitted from the transmitting apparatus, and comprises mode determining means for determining which of a normal receiving mode and a broadcast hindrance mode is effected, and reproducing means for reproducing the high quality signal when the normal receiving mode is effected while reproducing the low quality signal when a broadcast hindrance mode is effected.
According to the above arrangement, trouble in receiving radio wave due to a multi-path fading or the like can be reduced by a simply arranged radio wave receiving unit provided in the receiving apparatus.
According to the present invention, there is provided a transmitting apparatus for transmitting broadcasting digital data in a digital broadcasting situation, comprising high quality signal creating means for creating a high quality signal based on broadcast digital data, low quality signal creating means for creating at least one low quality signal based on the same broadcast digital data so that the quality of the low quality signal is relatively low with respect to that of the high quality signal, and transmitting means for transmitting the high quality signal created by the high quality signal creating means and the low quality signal created by the low quality signal creating means so that one of the signals has a time lag relative to the other.
The low quality signal creating means may be arranged to create a plurality of low quality signals different in quality from one another, and the transmitting means may be arranged to transmit the plurality of low quality signals as well as the high quality signal so that each of the low quality signals and the high quality signal has a time lag relative to another.
The transmitting means may be arranged to transmit the low quality signal before transmitting the high quality signal. Further, the high quality signal is transmitted by using a first channel band while the low quality signal is transmitted by using a second channel band of which width is narrower than that of the first channel band. Further, when there are prepared a plurality of low quality signals, the high quality signal is transmitted by using a first channel band while each of the low quality signals is transmitted by using a second channel band of which width is narrower than that of the first channel band. Furthermore, when there are prepared a plurality of low quality signals, each of the channel bandwidths is different from one another.
According to the above arrangement, broadcast data can be transmitted and received at a satisfactory quality level from a practical viewpoint by a simple arrangement, with the result that a number of programs simultaneously broadcast can be increased, and cost for system management per unit program can be reduced. In addition, since the frequency band for broadcasting can be effectively utilized, it is possible to take the advantage of system of time diversity that investment in providing gap filler or the like can be minimized while broadcasting can be carried out.
According to the present invention, there is also provided a receiving apparatus for use with a transmitting apparatus and receiving and reproducing a signal transmitted from the transmitting apparatus in which a high quality signal is created based on broadcast digital data, at least one low quality signal is created based on the same broadcast digital data so that the quality of the low quality signal is low with respect to that of the high quality signal, and the high quality signal and the low quality signal are transmitted so that one of the signals has a time lag relative to the other. The receiving apparatus is characterized by comprising mode determining means for determining which of a normal receiving mode and a broadcast hindrance mode is effected, and reproducing means for reproducing the high quality signal when the normal receiving mode is effected while reproducing the low quality signal when the broadcast hindrance mode is effected.
According to the above arrangement, also broadcast data can be transmitted and received at a satisfactory quality level from practical viewpoint by a simple arrangement, with the result that a number of programs simultaneously broadcast can be increased, and cost for system management per unit program can be reduced. In addition, since the frequency band for broadcasting can be effectively utilized, it is possible to take the advantage of system of time diversity that investment in providing gap filler or the like can be minimized while broadcasting can be carried out.
In the receiving apparatus set forth above, the mode determining means may be arranged to detect transition going to bring about from the normal receiving mode to the broadcast hindrance mode, and when the mode determining means detects the transition going to bring about from the normal receiving mode to the broadcast hindrance mode, the high quality signal and the low quality signal are processed and reproduced so that the transition is effected smoothly in terms of quality of the high quality signal and the low quality signal. Further, the mode determining means may be arranged such that, under condition that the broadcast hindrance mode is effected, the mode determining means does not determine that it is allowable for the normal receiving mode to take place until the high quality signal is successfully received for a predetermined time duration or predetermined number of frames, and the reproducing means may be arranged such that, when the mode determining means determines that it is allowable for the transition to the normal receiving mode to bring about, the reproducing means reproduces the high quality signal. Conversely, the mode determining means may be arranged to detect transition going to bring about from the broadcast hindrance mode to the normal receiving mode, and when the mode determining means detects the transition going to bring about from the broadcast hindrance mode to the normal receiving mode, the high quality signal and the low quality signal may be processed and reproduced so that the transition is effected smoothly in terms of quality of the high quality signal and the low quality signal.
Further, in the receiving apparatus set forth above, the reproducing means may be arranged such that, if the low quality signal is not received but the high quality signal is received under condition that the broadcast hindrance mode is effected, the reproducing means creates low quality data from the high quality signal to recover the reproduction of the low quality signal.
According to the above arrangement, also broadcast data can be transmitted and received at a satisfactory quality level from practical viewpoint by a simple arrangement, with the result that a number of programs simultaneously broadcast can be increased, and cost for system management per unit program can be reduced. In addition, since the frequency band for broadcasting can be effectively utilized, it is possible to take the advantage of the system of time diversity that investment in providing gap filler or the like can be minimized while broadcasting can be carried out.
When the transmitting apparatus of the system according to the present invention is arranged, the high quality signal creating means, the low quality signal creating means and the transmitting means may be provided in the same broadcasting station. Alternatively, the high quality signal creating means may be provided in a broadcasting station, and the low quality signal creating means and the transmitting means are provided in a broadcasting/communication satellite.
According to the above arrangement, the system can be managed with a flexibility. Therefore, if specification change is requested upon constructing the communication system, it is possible to respond to the request with ease and management cost thereof can be reduced.
Further objects and advantages of the present invention will be apparent from the following description which is given with reference to the accompanying drawings wherein preferred embodiments of the present invention is clearly shown.